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354 CINEMATOGRAPHIC ANNUAL
saw teeth, of course, depends on the shape of the original sound wave together with what distortions have been introduced into the system. The higher frequencies show as peaks close together like a fine tooth comb and the amplitude or width of the peaks measured across the track, is proportional to the loudness of the original sound.
Microphotographs of serrated edge or variable area sound track are shown in Fig, -4.
The power for moving the film past the optical system at a constant rate of speed of 90 ft. per minute, is supplied by a synchronous motor from the power source which supplies the camera motors. In ordinary studio operation, all of these motors are thrown across a three phase central station power supply without any intermediate compensators. A gear mechanism is used to reduce the speed of the motor so that it will drive a sprocket for pulling the film from the supply magazine, the recording drum, through a special compensating mechanism, and the take up on the film magazine. If there were no variation in different grades and stocks of film, no compensator would be necessary. Every different type of film, however, seems to have varying amounts of shrinkage and it becomes necessary to provide a means of overcoming this and still maintain constant speed past the exposure light.
The operation of the compensating mechanism for type R-3 recorder is shown schematically in Fig. 5. The synchronous motor drives the fly-wheel through a pinion and gear. The gear is attached to the flywheel by means of a spring coupling. On the same shaft with this flywheel is a belt drive for the take-up, cone "C" and sprocket "S" which pulls the film from the film magazine at constant speed. This shaft is also arranged so that it floats on a supporting arm. Pressure is exerted on this arm by a spring which forces the cone "C" on to an idler "A" and causes the idler to revolve by friction. Pressure from the cone "C" is transmitted through the idler to the cylinder "B" which is also driven by friction and causes the recording drum "D" to revolve. It will be noticed that the speed of the drum is determined by the position of idler "A", if it is in position "n", the recording drum will revolve slower than in position "m". The position of the idler is changed by a lever which tilts the idler and causes it to change its position on the cone "C". The tilting lever is controlled by a compensating roller. Thus, if the